[#BEAM-1196] Latitude shift for non-WGS84 datums

BEAM

Latitude shift for non-WGS84 datums

Created: 09/Sep/10 03:57 PM Updated: 29/Mar/11 02:24 PM Due: 09/Sep/10

Component/s:

General

Affects Version/s:

4.7, 4.7.1, 4.8

Fix Version/s:

4.8.1, 4.9

File Attachments:

None

Image Attachments:

1. MODIS_and_MERIS.png
(910 kB)

Description

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For geo-referenced products (map-projected to a well-known CRS), a latitude shift (~20 km, depending on datum) can be observed if the the underlying projection uses a datum different from WGS84.

From Gerardo (see attachment):
"While working with the reprojected data, I saw a big shift (~25km) between the MODIS data and the reprojected MERIS/VGT/AATSR data (check the attached quicklook, on the left MODIS, on the right MERIS), which is a problem since we cannot compare out Albedo Inversion with the MODIS products we're using as a reference."

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Change History

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Norman Fomferra added a comment - 09/Sep/10 03:59 PM

In CrsGeoCoding constructor:

        if (mapCRS instanceof DerivedCRS) {
            DerivedCRS derivedCRS = (DerivedCRS) mapCRS;
            CoordinateReferenceSystem baseCRS = derivedCRS.getBaseCRS();
            setGeoCRS(baseCRS);
        } else {
            setGeoCRS(DefaultGeographicCRS.WGS84);
        }

==>

        if (mapCRS instanceof DerivedCRS) {
            DerivedCRS derivedCRS = (DerivedCRS) mapCRS;
            CoordinateReferenceSystem baseCRS = derivedCRS.getBaseCRS();
            setGeoCRS(baseCRS);
        } else if (gtDatum instanceof GeodeticDatum) {
            setGeoCRS(new DefaultGeographicCRS((GeodeticDatum) gtDatum, DefaultEllipsoidalCS.GEODETIC_2D));
        } else {
            // Fallback
            setGeoCRS(DefaultGeographicCRS.WGS84);
        }

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Thomas Storm added a comment - 13/Sep/10 02:05 PM

Fix is necessary, but not sufficient to solve shifting problem.

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Thomas Storm added a comment - 13/Sep/10 02:20 PM

We have found that the MERIS data have been reprojected using the wrong CRS. For the MODIS Sinusoidal projection, there are two CRSs available, in particular SR-ORG:6842 (http://spatialreference.org/ref/sr-org/6842/) and SR-ORG:6974 (http://spatialreference.org/ref/sr-org/6974/), with the latter being a corrected version of the first one; the home page quotes:

"This is a corrected version of SR-ORG:6842, describing the global sinusoidal projection used by many MODIS land products. The projection uses a spherical projection ellipsoid but a WGS84 datum ellipsoid. Not all projection software recognizes the "semi_major" and "semi_minor" arguments to Sinusoidal projections. If yours does not, you may be happier using SR-ORG:6842 and coping with the fact that it may introduce datum-shift errors down the road. This is a variation on SR-ORG:6965 that uses the authalic mean radius of the Earth for projection rather than the major radius, in keeping with files distributed by NASA/USGS."